Flotation is a known process for separating valuable minerals from waste material, or the recovery of finely-dispersed particles from suspensions in water. Typically, an ore as mined consists of a relatively small proportion of valuable mineral disseminated throughout a host rock of low commercial value (gangue). The rock is crushed or finely ground so as to liberate the valuable particles (values). The finely-ground particles are suspended in water, and reagents may be added to make the surfaces of the values non-wetting or hydrophobic, leaving the unwanted gangue particles in a wettable state. Air bubbles are then introduced into the suspension. A frother may be added to assist in the formation of fine bubbles and also to ensure that a stable froth is formed as the bubbles rise and disengage from the liquid.
In the flotation cell, the values adhere to the bubbles, which carry them to the surface and into the stable froth layer. The froth discharges over the lip of the cell, carrying the values. The waste gangue remains in the liquid in the cell and is discharged with the liquid to a tailings disposal facility.
In some applications, particularly when flotation is carried out in tall columns, the froth layer can be relatively deep, of the order 1 to 2 metres. The particles in the froth can be from several sources. Most are hydrophobic particles, the values, which are attached to the bubbles. In addition, liquid is entrained when the bubbles rise from the liquid layer into the froth layer, and this liquid can contain high concentrations of the gangue material, which can pass out of the flotation cell with the valuable material, and accordingly will lead to a reduction in purity of the product. To reduce the mass of contaminating gangue material in the flotation product, a stream of clean wash water can be introduced into the rising froth, thereby providing a net downflow of water in the froth, which flushes out the particles of gangue. It is advantageous to provide a means to introduce wash water in an efficacious way, as uniformly as possible across the cross-section of the flotation column. Devices that are in current use are relatively simple, consisting of horizontal tubes or pipes with small holes drilled at regular intervals from which jets of water issue, into or on top of, the froth. The holes may be drilled in a line along the bottom of the pipe with a pitch of 50 to 100 mm typically, so that the water jets project vertically downwards. Alternatives are known where the jets project in the horizontal direction or at an angle of 45° to the vertical. Another form of washing unit is a horizontal tray suspended over the top of the froth, from which wash water passes through an array of small holes drilled in the base of the tray. In this case, the water jets project vertically downwards. Whether the holes are formed in the base of a horizontal tray, or in the walls of cylindrical pipes suspended above or within the pipes, a large number of holes is required, to deliver the desired flow rate of wash water. Many holes are needed because of a desire to distribute the wash water as evenly as possible across the flotation column. The optimal distribution would involve essentially an infinite number of injection points but such an arrangement is ruled out for reasons of practicality. Accordingly, designers have adopted the strategy of providing a multiplicity of wash water streams, the number of streams being a balance between the desire to keep the spacing between them to a minimum, while keeping the diameters of the exit holes or orifices as large as possible, to reduce the probability of blockage by particle deposition. Cost is also an issue because the greater the number of holes the larger the manufacturing cost.
The water that is available in mineral concentrators and mills is usually process water that has been recycled after passing through thickeners or settling ponds, and it frequently contains particulate matter that can block the small holes in the wash water distribution systems. Further, processes are known in which hydrophobic particles are deliberately introduced in the wash water, so that they may be captured in the froth layer. Such processes are particularly applicable to particles that are larger than those normally treated by flotation, so large that it is difficult for them to transfer into the froth from the underlying liquid layer. These larger particles settle rapidly in the wash water. The problem is particularly vexatious when it is desired to operate at low wash water flow rates, because under these conditions, the velocity of the water in the distribution pipes is insufficient to keep the particles in suspension and they fall to the bottom of the pipe and accumulate to form a bed of sediment that blocks the small exit holes. Changing the location of the exit holes does not prevent blockage, but merely delays the onset of blockage for the time necessary for a the level of the bed of particles that have sedimented out of the incoming wash water stream, to reach the location of the exit holes.
Throughout this specification the term “wash water” is used to designate the liquid introduced into the froth in this manner, whether it is used for “washing” or for conveying course particles or other matter.
Another factor that must be taken into account is the effectiveness of single streams of water, in the form of essentially cylindrical jets, in washing the froth. Although froths are known that are upwards of one metre in depth, it is also common for froths to be no more than 100 mm deep. When a vertical jet is introduced into a froth, it initially creates a region in the vicinity of the entry point which has a much higher liquid fraction (volume of liquid as a fraction of the total volume of liquid and gas bubbles) than the bulk of the froth. As the wash water flows downwards, it also tends to spread horizontally, until at some distance below the injection point the liquid fraction in a horizontal plane across the column cross-section is essentially constant. We have found that with vertical cylindrical jets the vertical distance required for a constant distribution to be reached is quite large, of order 0.5 to 0.8 m. Thus in shallow froths, much of the wash water will pass through the froth layer to the liquid layer beneath, without providing the desired washing action in the froth.